Think of all the problems with Tekkit Classic. You've probably grown used to them. There are countless exploitable bugs that players use to cause trouble, from duping items to crashing servers. Complex factories cause problems. Servers have to be restarted frequently.
What if Tekkit were nearly as reliable as vanilla CraftBukkit?
I can't fix everything. Some mechanisms are programmed so horridly, a complete rewrite of an entire mod would be necessary to fix the numerous problems that it presents. However, I have taken the time to rewrite, patch, and amend quite a bit, making Tekkit far more reliable than it used to be. I've even added some features to fix generic problems that even exist in vanilla Minecraft.
Download
Installation instructions and general information
Report bugs, request features
GitHub
Old versions
List of resources/links
Before you get too excited, here's a list of things that you must not do while using this fix:
Do not use another plugin to eliminate lag. The common techniques used in these plugins are destructive: they can cause problems for players and even maps. They will not help with lag while ForgePlugin is installed anyway, and may even conflict.
Do not clear entities globally. If you need to clear entities, use a reasonably small radius. This rarely helps fix lag anyway. ForgePlugin monitors entities; if it detects that they might cause lag, it will remove them in the least destructive manner possible.
Do not clear items or kill all mobs globally. These are entities, so the same rules apply as above.
Do not expect an item that is banned by default to be fixed. That's why it's banned.
And here's what you should do:
Do disable the built-in chunk cleanup timer and use a cronjob instead if your server is on Linux. The timing will be more consistent.
Do save the map at least every five minutes if you expect people to explore using flight, or your server will crash when it saves. This applies to vanilla, too. This is not automatic to prevent conflicts with backup systems.
Do report bugs that still when work using ForgePlugin.