XDA is the famous Android OS community about development and hacking. The most interesting part is the forum with many active users, where you can find threads about various models, from old ones to new.
Some of them have developed scripts/mods that became popular and allows to improve features and better performances (naturally is implicit that your device must be rooted).
CPU Editor allows to set various cpu parameters (Condition, Max and Min Freq, Governor, Priority)
AGPS Patch patch for devices that have issues with gps fixing, it allows to lock sats more rapidly
V6 SuperCharger | 3G TurboCharger | Die-Hard battery calibrator the first is for RAM optimization (useful when you have devices with less performant hardware), the second allows to improve 3G/2G data connection, the third to calibrate battery when you change ROM
Mount2SD a control script that allows to move contents to SD (apps and apps data, etc…)
Seeder Entropy Generator provides significant lag reduction, especially in old devices. Available as APK format ready to be installed
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