Hi all! This is my first bukkit plugin.
What does it do? It simply removes the max player limit you or your hoster has set. To try avoid frequent server crashes, it checks how much free memory your server has, and if it is less than the amout (in MB) you specified in config.yml it kicks the player with a custom message.
It's in a very early stage, by now it doesn't provide any command, but you can enable and disable it at any time editing the config file without the need to reload every plugin.
How to install it:

Download it from the provided link
Copy PreventFull.jar in you plugins folder
Start your server
Edit /PreventFull/config.yml

Download link: http://dev.bukkit.org/media/files/541/391/PreventFull.jar
How to configure it:
Open /PreventFull/config.yml

general: 
    enabled: true # quite obvious
memory:
    min: 0              #minimum amount of memory the server must have to let someone log in, 0 for no limit
messages:
    banned: You are banned from this server!            #What should the server tell the player when it doesn't let him in (when he is banned, and when memory is low
    memory: Not enough free memory to let you in!

Known bugs:

If you banned someone's ip address, that ip address will still be able to log in the server (not if player name is banned too). You should use something like Easyban to solve this.

Todo list:

Add ingame and console commands.
Add permissions support.
Solve the 'banned-ips' issue
Find a better way to check if someone's banned

PreventFull