Android only: The SMS messaging app included with standard Android phones is decent, but it could do so much more to make texting easy. ChompSMS does those things, including quick replies from home or lock screens, a clever widget and more.
From the get-go, ChompSMS is much more customisable than the standard Messaging. You control the colour of notifications, the look of the app (with skinning even possible), whether contact icons appear in messages and how ChompSMS gets your attention. There's also a group messaging feature not available in most SMS clients, and if your texting rate was some kind of rip-off scheme, you can even buy SMS credits through Chomp for independent use.
Where ChompSMS really one-ups Messaging though, is its ability to hit you with SMS messages on the home screen, or even light up your locked screen for a few seconds to show you your most recent ping, iPhone-style. The app's home screen widgets offer a similar quick read of your messages and ability to quickly reply without even launching the app. You can check a privacy switch to prevent sender and message text from being shown and require an unlock code before anyone can use the quick replies — but if you're grabbing this app, you probably want to reduce the steps to SMS, not increase them.
ChompSMS is a free download for Android, though there's an ad-free version also for sale. You can grab it directly using the QR code at right or search your Market app.