MAME is a one of the originators in keeping old school arcade gaming alive. Created in 1997, the Multiple Arcade Machine Emulator is renowned for having support for thousands of rom sets. And now you can have limitless fun on your HTC EVO with MAME4droid.
Created by Seleuco, MAME4droid currently has over 2,000 games in its support list. For those who are already familiar with the rom scene, it will use only the "0.37b5" and the "GP2X, WIZ 0.37b11 mame romsets." They were nice enough to include a ClrMAME Pro dat file so that you can properly identify roms that will work.
While this all sounds daunting, the reward for some work is very clear in what is solid support that includes the ability to re-map buttons and even 2D hardware acceleration for Android 3.0 devices. One feature it lacks is save states, but those who have never used another implementation of MAME won't mind since arcade games do not have the ability to save like console games, anyway.
MAME4droid is currently free in the Android Market, so get out there and start playing all those classic games. My personal favorite is the 1982 Midway release, Spy Hunter. The absolute worst MAME game, due to me sucking at it, is Circus Charlie.
Now remember this is a grey area to many in terms of proper use and legality so please respect everyone's fun and do not discuss or share links to rom images or sites in the comments.