We've been doing a lot of application benchmarking and tuning at work in the last couple of months as we are prepare for a new major release. I've been involved in the Oracle side of things and have spent a lot of time tracking down expensive (or plain weird) SQL-statements and trying to optimize the Oracle databases we use.
After spending about 6 - 7 hours a day running my normal sqlplus scripts over and over trying different things, monitoring benchmark runs I got a bit bored of text output and had a quick google for some sort of basic v$whats_up GUI tool and found this little cool little app called Insider for Oracle, it a great app for doing spot-on monitoring, sort of like Spot light from Quest, but without the hassle. Insider tries to be a more proactive tool with round the clock monitoring, but for that, it's no good.
The thing I find it useful for is when we start a benchmark I can trail what's going on with the system. See new SQL statements, wait events, SGA usage, PGA usage, hot segments etc. Nothing I can't do from sqlplus but it makes my tasks a bit easier.
Price wise it's quite a bargain for what I use it for, the license is for each database on the application "workspace" (i.e. how many databases you want to look at at once). And for what I use it for just looking at one DB at a time, that works out pretty cheap, 599USD or so. Cheap as chips.
I've found a couple of bugs (and submitted bug reports accordingly) and I've even submitted a couple of enhancement suggestions.
Check out their company blog.