Is there any software I can run to speed up my MBP?

My Macbook Pro is runnign real slow these days. At first I noticed i had only 15 GB free on a 500 GB hard drive. Well, I deleted a lot and now have 70 GB free. But it still is running slow.

Any other thoughts? Any software I can run that might help? Similarly, how can I diagnose if some of the hardware is beginning to crap out? I bought this in 2009 so it's definitely possible. I'm running 10.7.5

To reach the minimum free space I'd ever run on OS X (15%), you'd need to hit 75GB free. I'd start by freeing up more drive space.

Download ONYX and run some of its maintenance tasks, like clearing the caches and running a permissions repair

Titanium's Software • Index page

After running Onyx - making sure your Trash is empty - deleting everything you have in the Downloads folder that is no longer required:

Just freeing up space at this point probably is not going to help just a whole lot as the free space is going to be fragmented along with your data being spread from one end of the drive to the other.

Options to defrag the drive
back up with Time Machine, then erase and reinstall OS X and restore from the TM backup when it asks during the reinstall
clone the drive with SuperDuper! and then clone it back
Buy iDefrag and defrag the drive.

In my testing, iDefrag provided a better increase in speed than the other 2 options - both in boot times and application launch times.

imho, and I readily admit, I am on the extreme edge regarding free space optimization, 15% only gives you enough room to run OS X - it is not going to keep OS X running "optimally".
I notice the OS begin slowing down at around 35% free space and no longer allow my system partition below 40% free space. If you've not really noticed yours slow down until it got this low, then 15-20% may be alright for you.

Another idea that may be contributing to this: how much stuff do you keep on the desktop?

(hint: correct answer is "nothing but the drive icon")

^^^^ true that

extremely bad habit

+1 Onyx. I used it yesterday and I have noticed a great improvement ever since. Just run the automation task on it.