My favorite is sms popup from the market. You can chose to have it light up your screen or not. Then you can reply directly to the message. I use it because I like the stock messaging app but I still want a popup like handcent.
Handcent has the pop-up setting that will 'wake' the screen... and it can also be set to "pop-up" over the lock screen, allowing you to read and reply to texts without having to unlock the device.