Multiple Worlds without running multiple servers

Over the time I've had Minecraft and had a server (since early Alpha), I've gone through many, many maps. Mostly all of them are above ground. I don't want to copy and paste them into one large map, as most of us playing on my server would like to keep them intact in their old worlds. Is there any plugin where I can implement multiple maps, both above-ground and nether, all on one server?

Otherwise, how does ServerPort work nowadays, and does that allow for this (without running 8+ servers on one computer)?

More info:

Using McMyAdmin Professional (newest version)
2gb memory (RAM), on an x86 intel-based Mac running OSX 10.6

 

i run bukkit with multi-verse, you will need permissions to run multi-verse. then its as simple as typing

/mvcreate <worldname> <maptype> (seedvalue)
<>required () optional

so /mvcreate survival normal Glacier
will create a normal world called survival from the seed Glacier
or /mvcreate hell nether
will create a nether world called hell that is of random seed

you could also import your old maps by typing

/mvimport worldname maptype

if you had a previous map called survival that was a normal map type

ex /mvimport survival normal



to make portals get a wooden pickaxe 2 blocks a couple of spaces apart +__+ then left click one block then right click the other, then type /mvpc portalname w:destinationworld

so /mvpc world2survival w:survival

once placed you would need one back
/mvtp survival to teleport to survival

now make a new portal structure, i like the 2 blocks for starters, you can change it once its done
once again left and right click blocks with the wooden pickaxe.
then type

/mvpc surv2world w:world

i run 6 maps on a 2gb box and can get 35 players, your mileage may vary with map sizes and plugins used

 

Thanks for all the information! I can now have my server at its best!