Another vote for DD-WRT. I used to use Sveasoft's Alchemy, but DD just flat out blows it away. As an added bonus, DD-WRT is truly free, unlike Sveasoft's crap where you have to buy a forum membership to get the latest 'public release.'
I'm using it for a lot of the same reasons as other folks.
1) Boosted power output (who needs those pesky, overpriced antenna upgrades?)
2) QoS - A must for being able to download torrents all day and night long without affecting VoIP traffic
3) I love the home/landing page that shows up before logging in. Gives a great at-a-glance view of what the router is up to
4) VPN Endpoint
5) Remote NTOP stats are a nice feature too. Get to see what kinda traffic is going thru the router from my Windows XP workstation
6) Haven't had a need to yet, but will probably take advantage of the VLAN support in the near future as well.
Definitely worth the couple of minutes it takes to download and install it!