Lion is out and about, though that doesn’t mean that every app has been updated to take advantage of one of the primary features: full screen apps. Some of the apps will be updated in due time (such as Chrome and Firefox), while others (like the extremely popular TextMate) might not. Maximizer attempts to solve this, albeit in a rather hacky way.
To use Maximizer, you have to install SIMBL and then put Mazimizer.bundle inside ~/Library/Application Support/SIMBL/Plugins (or in the same folder in /, if you want it globally available). I probably can’t help you get this installed; maybe read the SIMBL website if you get confused?
The install process, as well as the overall stability, will perhaps improve with help from saurik’s CydiaSubstrate.