Because it takes such amazing pictures, the iPhone 4 has become the pocket camera of choice for many users—myself included. The phone’s built-in camera is easy to use and always handy. What’s more, the App Store is loaded with a number of great third-party apps to help you make the most of your photos. And many of these filters will cost you very little—or nothing at all.

Apps like Adobe Photoshop Express, Hipstamatic, and Instagram deservedly grab a lot of attention. But I’m always on the lookout for image-editing apps that fly under the radar. Here are five I’ve used lately to tweak my mobile photos.

Pixel It from Mr. B. Ware actually does what other apps might try to fix—it adds pixelation to your photos. Ever wanted to make yourself into an avatar for an Atari 2600? With dozens of possible adjustments, the $1 Pixel It can make a creative mosaic or make your image look like it was taken with an old Apple QuickTake. (Remember those?)

The free Old Photo Pro from Deion Mobile provides a perfect example of a single-function app that does its job well. Take or load a photo, make some simple adjustments to brightness, contrast, tone and color, then get to some crumpling. The old paper texture and edges options make your picture look like it has been folded up and kept in grandpa’s wallet for a few years. It’s a cool effect—one I haven’t seen in other apps.

If you’d like to jettison Old Photo Pro’s ads, the app offers a $2 in-app purchase that does precisely that. More important, the in-app purchase also adds support for higher-resolution images.

Tandem Systems’ DXP Free is, as its name would suggest, free. This is an fun double exposure app that allows you to overlay two images into one. The controls are a bit quirky, and DXP Free doesn’t provide a lot of options for adjustment, so it’s best to plan ahead. Figure out what images you want exposed together, choose your composite effect, and then go for it. The final images are often surprising, but I found the best results when I kept it simple. A hi-res version of DXP is also available for $2.

Fotolr Photo Studio from Tiny Piece is one of the multitude of all-in-one photo offerings in the App Store. This free app lets you crop, color, and touch up all you like, which is all well and good. But skip that stuff and get to the Scene interface where the fun begins.

There are more than 50 cool and fun frames or scenes to put around your image, including an ample selection of holiday greetings—fast, fun and personalized cards, here we come. An uploader to Facebook and Twitter makes sharing that much easier. You can remove the ads from the app via an in-app purchase.

If you dream of turning your images into a classic Soviet-era style propaganda poster, iStalin from Eighth Art will do the trick. Be warned that the interface on this free app is downright frustrating. Still, no other app that I’ve come across does this, and the results are really cool once you get the hang of it. You overlay your image with pre-made text, stickers and paper textures to get that “for-the-motherland” feel. The app won’t appeal to everyone, but as a fan of this form of artwork, I find it to be a good addition to my iPhone. (iStalin is a universal app that runs natively on the iPad, too.)

I’m always looking to expand my creativity and post something new. These five apps give you a wealth of new options for your photos, without running up a huge tab at the App Store.

[Jeff Phillips is an educator, photographer and technology advocate. Find him on his blog or podcast.]