CyanogenMod 7 nightlies now available for Samsung Galaxy S II
Of course, in order to install the CyanogenMod ROM, you'll need to root your device, but once you have, you can either manually flash the ROM or have ROM Manager do all the heavy lifting for you. It should be noted that while this is listed as CyanogenMod 7 (which is usually based on Android 2.3.3 whereas CyanogenMod 7.1 is based on 2.3.4), this ROM for the SGS2 is in fact based on the latest Android version 2.3.4.
As always, keep in mind that nightlies are very early releases that are likely to have bugs. And, while there are a number of benefits to rooting, there are a few downsides, most notably that rooted devices can't watch rented movies through the Android Market. Hit the XDA link for instructions if you need them.