ATechVerse: Good Cydia Repo's

Apart from default repositories, I would recommend following Repo's for Cydia:

First source to install after iPhone/iPod/iPad Jailbreak
Main Apps: Installous and AppSync [Must Have Cydia Apps]. You will need to install Installous ONLY; AppSync will be automatically installed.

Best source for cracked Cydia themes and DreamBoard themes (Cydia Store themes)
Main Themes: Amplified HD, BoxorHD, Boxor HD – Ceramic White, blAze HD/iPad, Elite PRO HD, Glass Orb, iFlat, iFlat4 HD iPad, iNitsua Z SD/HD/iPad, inPulse for iPad, inPulse HD/SD, Kryptonite, LiveOS, Omega HD, Prestige-HD/SD, revi-krs HD+SD, Revolve HD/SD, SBHD,Snow Leopard and many more
Good source for games DLC and cheats.
Excellent source for cydia apps/tweaks as well.

Best source for cracked Cydia apps and Cydia tweaks (Cydia Store apps).
Main Apps: Action Menu Plus Pack, AndriodLock XT, Barrel, biteSMS, frash, Graviboard, Gridlock, iAcces, iFile, Infiniboard, Infinidock, Infinifolders, Lockinfo, Masks, Multifl0w, My Wi, SleepFX, YourTube 2 and many more.
Excellent source for cracked Cydia themes
Good source for games DLC and cheats

Repo hosts a good deal of ROM packs for Game Boy Advance, Nintendo 64 and NES which of course you can load on your emulators.
Excellent source for cracked Cydia Store apps/themes: e.g. Cycorder, Cyntact, CLASSified HD, FBUploader, iBlacklist, ScrollingBoard, Speed Intensifier and many more.
May cause Cydia loading errors

Others (decent but not necessary):

This repo contains a wide range of apps, mods and ringtones for your iPhone. This is one of those all-in-one repos that holds some highly popular and useful Cydia apps and tweaks. It even contains some new jailbreak tweaks for the iOS 5.
Excellent Italian source for Cydia apps/themes, e.g. Full Screen for Safari, GroupSMS,Home Page in Safari and Wifi Sync.

Not a very well-known repository but once you browse the content of this repo, you will find that it contains a wide range of pretty much anything. Apps, tweaks, utilities, addons, themes, ringtones, emulators, you name it. The number of content is not that high but it makes up for the quality.

Another repository that is rich in content. One thing that you will notice about this Cydia source is the wealthy selection of themes although there is no shortage of apps and tweaks to be found in here. We recommend that you install the iCauseFx repo icons after adding this source for easier package browsing.

When it comes to ringtones, themes and wallpaper, then you can't go wrong with this source. This repo specializes in hosting multimedia content rather than the usual apps and tweaks. Again this repo hardly has any tweak or app, so if those are what you are looking for, then you might want to look elsewhere.

Excellent source for free Cydia apps/themes: e.g. AndroidLock, Attachment Saver, FaceBreak, GameBoy emulator, iBlueNova, iRealSMS, My3G, MyWi, and NES emulator.
May cause Cydia loading errors

iAP Cracker: Great app cracks inApp Purchases and make it Free.
Examples for free inApp Purchases: Angry Birds (Mighty Eagle), Asphalt 6: Adrenaline, Jetpack Joyride, Line Birds, Plants vs Zombies and Tiny Tower.. .

ATechVerse