I downloaded a couple of free apps on my Blackberry and wasn't too happy with the functionality. Blackstar points you in the direction of way-points but it seems to get too easily confused. And the compass required you to move to give orientation. I also didn't like the idea of using my phone while riding, I have butterfingers and do look at the screen while in motion, so I wanted something with a rugged construction.
I ended up looking at some Garmin trail GPS's and found a decent price on a 60CSx without the Sirf chip-set ( I don't think you need it unless you do surveying). I hang it from a carabiner on the front of my Camelback and it has been quite useful in getting me back to the parking lot. The GPS, case, carabiner/clip & memory card came under $300 (All purchased separately).
If you want something cheaper that will get you back to the parking lot, you might want to look at the eTrex series. The 305, 605 & 705 are bike specific GPS's. A lot of the non-bike specific GPS still have bike mounting kits.