Recovery is not limited to one's Up-B. For many characters, there are many ways to augment the recovery.
Bowser
In theory, Bowser's Flying Slam (only in SSBB) could, if he grabbed onto a character in midair, propel him up, possibly to safety.
Captain Falcon
In SSBM, the Falcon Kick would allow Falcon to regain his 2nd jump in midair, and move him forward somewhat. This is not true for any other Super Smash game to date.
Diddy Kong
Diddy Kong has no supplementary techniques for his recovery.
Donkey Kong
Donkey Kong has no supplementary techniques for his recovery.
Dr. Mario
Dr. Mario can use the Cape (technically the Super Sheet) to propel himself forward a bit. Also, if he rapidly presses B during the Dr. Tornado, he rises up in the air.
Falco
The Falco Phantasm moves Falco forward, and should always be used to recover; indeed, that is its primary use.
Fox
The same goes for the Fox Illusion as for the Falco Phantasm.
Ganondorf
See Ganondorf: Recovery.
Ice Climbers
Belay is an incredible recovery move when both Climbers are present; however, it doesn't work otherwise. Use Squall Hammer and press B rapidly, and you will rise into the air. This is basically your sole recovery move when Nana has been Ko'd, but it works better when she is present.
Ike
Ike's Quick Draw can be charged, and moves him forward quite a bit. Use it to augment the poor horizontal recovery of Aether.