Well, it would depend on what kind of apps you want and/or need. But in my opinion, the #1 recommended app for ALL Droid phones is the AdvancedTaskKiller app. What it does is that it "kills" the apps that are currently running so that way your battery lasts longer. It's highly recommended to use the app if you're setting your phone down and it will be that way for a prolonged period of time.
You can find them on the android market under the free section. This will allow you to find the top free apps for your latest android phone. This is how I find the best android apps.
Android market place (market.android.com)
Obviously, The best place to for android apps is android market place. The official platform to download every app of android but not those which were suspended because for violating rules. Many good apps were suspended from Android market just because of policy issues even people liked those apps a lot but still they did it. On the whole one can find more than 85% android apps on Android market place.
No, Android Market apps are only for Android phones. For an iPod Touch, you would have to use apps from the iTunes Store.
You go to the android market
I recommend you AppXoid, they publish great reviews of new Android Applications and Games. If you are looking for an alternative market, I recommend you Getjar. You can find cheap applications on this market.
There is no exact figure available on Internet to know the exact number of android apps but according a professional estimate there are 3,75000 apps available on android market. In August it is expected that android market will be having more than 4,00000 apps.
Android jailbreak
Yes they have tons of apps and most of them are free and very easy to use. They have apps for games, books, music, and a lot more the android market apps have some of the best apps you will find out there and most of them don't take a lot of memory in your phone so you can get tons and they also have apps for free ring tones as well.
Google play or android market
they say about over 50,000 apps
market app.