One that shows the soul level, and what gem was filled, instead of the generic message that pops up when you kill something. I did a quick Google search, but couldn't find anything. Anyone know of one?

Not that I've heard of. The best thing to do is carry around a variety of empty gems, really sucks to fill that expensive grand soul gem with a petty soul! And the Black Star, a must-get of course.

@INV2: I've been looking all over since I posted this, and I haven't found anything that resembles what I want. My best guess is that someone would need the construction set to do a mod like this....?

Read a guide. That's the only way. After a while you get a feel for what things get into what soul gems.

Just get the Black Star and start murdering guards. Black Souls are as good as Grand Souls and you can grab them from any humanoid.

Try Smart Souls:

http://forums.bethsoft.com/topic/1300172-rel-smart-souls/

@Xealot42: Perfect! Thanks for the help.