I'm not a heavy app user and I rarely download anything new to my phone. So when I find something I like I get very excited and want to share it with everyone.
GymPact - I do better both mentally and physically when I workout on a very regular basis and I'm extremely loss averse, so this app works great for me. Basically, you commit to a certain number of workouts per week (minimum of 1/2 hour to count, no more than one workout per day) and bet a dollar amount that you'll do it. I'm committed to 4 workouts per week at $5 per workout. If I miss a day I pay $5 into the pot and it gets distributed to other people who did make their workouts. Because the pool is huge, you only get paid about 25 cents per workout, but the real motivator is avoiding loss so it works out well. So far I've made $15 and lost $5. The real benefit is that this is the most consistent I've been about the gym in ages. You can sync it with RunKeeper for outdoor activities and I do this to log my runs.
Naturally I obsessively read internet reviews before giving this app access to my Paypal. There are a lot of people who have had bad experiences (common complaints are problems with syncing to RunKeeper, issues with losing or not getting credit for workouts, trouble cancelling the service or changing workout commitments). I dove in anyways because I really wanted to try it (so why did I read all those reviews? I don't know) and I figured I could deal with the hassle of cancelling if it was a bust.
So far (11 weeks in) I've had zero problems with it. Every once in a while it will fail while trying to upload my workout but I hit retry and it goes through. That might just be the spotty internet connection at my gym. I've adjusted my workout commitment on weeks where I had a vacation or just a crazy schedule. As long as you do it before the week starts, it's easy peasy. It uses GPS to make sure you are actually at the gym when you check in, so you need to make sure your gym is registered. I've forgotten to check in a couple times, which drives me completely nuts but that's entirely my fault and I can't blame it on the app, sadly.
I'm sometimes (deeply) annoyed that I can't just skip the gym after work, but that's the whole point, right? I almost never regret a workout once I've done it, so it's just getting myself there that's difficult. This helps me get there.
RunKeeper - As a basic, free app, RunKeeper is pretty good. I like having the option of tracking my outdoor runs and it syncs with GymPact flawlessly so I get credit for them. Some people have been annoyed that you can't listen to music while using the app but I've never had an issue with this. I just open up my Pandora before I start RunKeeper and then they both run simultaneously. There are also playlist options but I still haven't bothered putting any music on my phone. It seems to automatically pause while I'm waiting at stoplights because while the overall time is still running, those pauses don't factor into my pace, which is nice. That said, I have issues with it being less than accurate. I've identified a couple of places along my usual routes that seem to be black holes. I'll run one block and suddenly RunKeeper thinks I've gone 2 miles further than I have, which has me running a 5 minute mile pace. I'd love to take the credit for that, but it's never going to happen. I've learned to avoid certain blocks (not sure what is up with that or if I'm crazy thinking it's that consistent) and my estimates tend to be more accurate. I have no idea if this is something to do with living in a densely populated city with lots of signals bouncing around or if it's inherent in the app. I'm not a serious runner, so the time/distance tracking is for fun and I can live with inaccuracy, but now I'm tempted to get a Garmin or something similar. Data is addictive!
SkyView Free - Just for fun. You get to see an overlay of stars everywhere when you open this app. It's set up so you can "see" straight through the Earth, so you can figure out which constellations are where at any time of the year. Bonus - you can take pictures of people with halos of stars and planets. This has no practical application but it's a nice party trick, if you happen to go to really geeky parties.