Sorted in reverse chronological order.
(9/27/05) On your turn, as a play, you can play two (or more) cards of the same color that sum up to the number of the card at the top of the discard pile. The 2+ cards thus summed to not have to be the same color as the top card. You cannot use zeroes as a sum, and you can't sum with / to word cards.
(9/20/05) Reverses matched on a stack have game effects. (Didn't seem to come up very often.)
(9/13/05) The "Deadly 1s". When you play a one, you can do one of two things.  Name a person, who must then draw a card and keep it face-down in front of them. Said card is not passed or played, it just gets added to your score when someone goes out.  Alternatively, when you play a 1 you can remove a face-down card that's in front of you.
(8/23/05) Matched Skips have game effects. In normal play, a matched Skip skips more people. In a stacking progression, a matched Skip delays the blast for more people.
(8/16/05) You can mix Draw 2s with Wild Draw 4s. If a D2 is played following a WD4 the D2 has to match the color named by the previous player. Otherwise, a D2 can be dropped onto any other word card, regardless of color. The stacks tended to grow quite a bit bigger with this house rule, which was the desired effect.
(8/02/05) Any 'Draw 2' or 'Wild Draw 4' played as a match increases the count on the stack. We found that the biggest application of this was to dump more cards on the poor sod that had to draw at the end. A very enjoyable Rule of the Week and quite possibly our most spiteful.
(8/02/05) This week we did a variant on the classic "show me 5's" rule: If someone plays a 5, the other person has to show their hand, they can block with a 5, etc. The twist: other players can match on an exposed player's cards. Said matches become part of their hand, count against them if someone goes out, etc. Color crazy match-ups still happen only in the bowl, though. The folks enjoyed this one.
(7/26/05) The starting card is a "screwball"; it can be played as any card. (We used "screwball" because the term "Wild" was already in use.) When you play it, announce what it is: Red Draw 2, Wild Draw 4, Normal Wild, Blue 6, etc. We discovered that you can always match the screwball (because it can be anything), so it's the best card to have in your hand as the last card. If you stupidly decide to leave it in your hand when the round ends, it counts for 100 points. This was one of our most popular house rules.
(7/21/05) We did "Crazy 8s" this week. If you play an 8, you can (at your option) dump all the cards in your hand into bowl and draw 8 new cards. (The original incarnation of this rule of "drawing a new hand" was a bit flawed, so we modified it a bit when Thursday came around.) It was occasionally useful, especially for people with humungous hands.
(7/05/05) The "random rule of the week" this time was: you can play a zero of the right color (matches the color of the last card played or the last color announced) on a stack to cancel it outright; no cards drawn, no more stacking. It didn't see a lot of action on Tuesday, but Thursday it came into play quite a bit.
(7/05/05) This kicked off our "random rule of the week" program; we draw a card to see which ROTW we should play with. The one that came up this week was: you can play 2's as Draw 2's on a stack and 4's as Draw 4's on a stack, as long as the color matches. A 2 or 4 can't lead off a stack, nor can you match a 2 on a Draw 2. Also, we said you can't play a 4 of one color on a 4 of a different color on a stack.
(6/21/05) You can play a zero on a stack. This causes hands to trade and the person you passed to now has to draw cards.
(6/02/05) We didn't have an official RotW this time, but for Thursday we decided to dust off an old chestnut and do the "show me 5's" rule again (Casey's variant).
(5/24/05) George picked the rule this week: Every Skip or Reverse played on a stacking progression adds 1 to the count. It tended to make the spreads (and spread reversals) much heavier.
(5/17/05) Clark picked the rule this week (under heavy lobbying from Woody): if it's you're turn and the only playable card you have is identical to the top card, you must play it as a match, and draw. Everyone seemed to get hurt by this at least once (more for Joey).
(5/10/05) Jeff picked the rule this week: no retroactive matching. A challenging variant. Lots of people ended up hovering right by the discard pile, waiting to catch a glimpse of the card that was about to be played so they could match the previous card.
(4/26/05) Ryan picked the rule this week: no starting card, the dealer just plays any card out of his hand and it has immediate game effects. The vindictiveness started early, as lots of word cards were played right off the bat.
(4/26/05) Joey picked the rule this week. Another variation on the "showing hands" idea, if 1 or more people draw 8 or more cards from a stacking progression (not just because they have to draw because they don't have a playable card in their hand on their turn), everyone at the table holding 3 cards or less has to lay down their cards on the table. (Just like the 5s, any cards drawn thereafter can stay in their hand and if the hand is swapped via a 0 or a 7, they become concealed again.)
(4/19/05) Woody picked the rule this week, the "swap seats on a 4" rule. It follows the same mechanic as the 7's or the 5's (play, block, up ante, etc.). On a successful play, the 4-player can change seats with someone else at the table and the new person next to them gets to go next (so you can end up skiping people). We quickly discovered that it helps to have one comfy chair at the table that everyone wants to sit in. We also discovered that we get more exercise with this rule in play.
(3/29/05) Casey picked the house rule this week. A variation on the "show me 5s" rule from last week, if the blocker is the last one to play a 5, the person who played the first 5 has to show their hand.
(3/29/05) Mark picked the "show me 5's" rule: If someone plays a 5, they can pick a person who has to show their entire hand on the table. The person picked can block with a 5, and the original player can "up the ante" with another 5 (just like with 7's). The person who showed their hand has to keep it on the table, but if they draw cards afterward (i.e. from a Draw 2 or Wild Draw 4) they can keep those concealed. The hand becomes "unexposed" if one of two things happens  all the exposed cards get played out, or  the hand changes owner via a 0 or 7 being played. In practice, people with the fewest amount of cards (esp. ppl with "Uno") were made to show their hands.
(3/29/05) It was Ben's turn to pick the rule of the week. He, however, declined to pick a new one so we played with our "standard" rules.
(3/29/05) This week marks the advent of our attempt to test out new rules by giving each player the chance to pick a rule for a week. Eric started with the "play a straight" rule: If you have a sequence of at least 3 cards, you can play them all in ascending order. You can't match with a straight and only the top card has effects. We discovered a suprising number of little quirks that needed to be worked out to get this rule to gel with all the others we use.