In response to a request for games that are relatively easy to learn (max 1 page rules), offer some measure of fun/excitement (for "older" people), have some degree of replayability (i.e., some strategy along with the luck), are quite quick (15 minutes or so per game) and allow people to join or leave quite readily (for kids/dinner etc!), several possibilities suggest themselves:

Try some of the Cheapass Games (www.cheapass.com). Give Me the Brain, Lord of the Fries, and Bleeding Sherwood come to mind. Gibson's publishes the Sherlock Holmes Card Game which goes up to eight, and TimJim's Mystic War is also good light fare, though not so quick as 15 minutes. AlphaBitz from Wizards of the Coast is a word game that qualifies.

Avalon Hill's Wrasslin' can be used with eight, though normally for two. Muenchhausen from ... I don't remember which German company makes it. This is excellent eight-player stuff and fits your criteria very closely (just a bit longer than you want) - you can read the rules at http://www.gamecabinet.com/rulesText/Munchhausen.txt. Which reminds me: there's a game coming out from Hogshead Publishing, UK, this summer: the Extraordinary Adventures of Baron Munchausen which will fit this bill very well - look for it! And that reminds me of Once Upon a Time from Atlas Games, which would also work for your purposes. To be honest, though, when we get to eight people, we usually split in-to two groups: four-four, five-three, or even six-two, depending on what people want to play.