1. Mapping
2. Conversion
3. Clients/Games to Practise with
4. Skinning
5. Extra Utilities
6. Guides
7. Mirrors

1. Mapping:
   
   ArrowVortex: https://arrowvortex.ddrnl.com/
   ArrowVortex is the preferred Stepmania/FFR Beatmap creation tool. It also supports .osu format and autoconverts ogg to mp3. This is probably your best friend for mapping if you don't want to use the default editor.
   
   OpenChart: https://github.com/OpenChartProject/OpenChart
   OpenChart is an opensource, cross platform tool for beatmap creation. Basically, open source ArrowVortex, it supports Stepmania, Quaver, and osu!mania.
   
   Default Editor (osu!editor): Built in.
   Originally created for osu!standard beatmap creation but works well enough for mania that most people use it.
   
   osu! Rate Changer: https://osu.ppy.sh/community/forums/topics/640981
   Takes any beatmap and changes the rate.
   
   TimingAnlyz: https://www.dropbox.com/s/ad02qxlh98pjnaz/TimingAnlyz%200.32.4.zip?dl=0
   Finds offset and BPM of any mp3 file.
   
   Mapping Tools: https://mappingtools.seira.moe/
   A really useful osu! mapping toolkit.
2. Conversion:
   
   BilliumMoto's o!m to Stepmania Conversion Tool: https://github.com/BilliumMoto/OMiSM
   In my opinion this is the best tool for converting osu!mania to Stepmania charts. Check out the original forum post here: https://osu.ppy.sh/community/forums/topics/664319
   
   Raindrop Engine: https://github.com/zardoru/raindrop
   Not actively maintained anymore but still works all the same. Allows for the importing of Stepmania files, osu!mania charts, BMS, o2jam, and bmson. Probably the best example of a universal VSRG.
3. Clients/Games to practise on: (This assumes osu!mania is your main VSRG)
   
   Quaver: https://quavergame.com/
   Really great open source VSRG on steam supporting 4k and 7k. This is probably the go-to mainstream client for SV maps at the moment since it supports some insane BPM changes. Also supports osu!mania, Stepmania, and Malody maps AND ranked judgement scaling!
   
   Etterna: https://etternaonline.com/
   At the moment this is the main Stepmania client/server for keyboard Stepmania players. It has ranked judgement scaling, ranked specific speed changes, and overall really customizable gameplay. 10/10 recommend as a main game, if not a main then a go-to practise client.
   
   YAVSRG: https://github.com/percyqaz/YAVSRG
   Interlude: A VSRG client supporting osu!mania and Stepmania with plans to add BMS support in the future. Developed by an osu!mania player to be a client that allows for even more creative mapping. It's SV capabilities are on par with/exceed Quaver's.
4. Skinning:
   
   osuskinner: https://osuskinner.com/
   I don't really suggest using this that frequently for mania as its asset selection is a bit limited, but it works well for generating a template and has decent interface elements.
   
   skinship: https://tutorial.skinship.xyz/
   Good tutorials and resources for skinning (Suggested by Jianshyy.)
5. Extra Utilities:
   
   KeysPerSecond: https://github.com/RoanH/KeysPerSecond
   A really awesome KPS meter made for osu!. Can be used as a streaming element or just as something to gauge your gameplay. See original forum post: https://osu.ppy.sh/community/forums/topics/552405
   
   Remove Beatmap Backgrounds Tool: https://osu.ppy.sh/community/forums/topics/365196
   So Background Dim exists, but why use a built in game option when you can rid your computer of scantily clad 2d women? Seriously, I don't recommend using this unless you really need to, but just make sure you are prepared to see ANGGGGGGGGGGGGGGRYYYYYYYYYYYY Peppy.
   
   osudaily: https://osudaily.net/index.php
   osu! stats tracker. Shows user statistics in great detail.
6. Guides:
   
   ERA Zenith's Comprehensive How-To-Accuracy Guide: https://osu.ppy.sh/community/forums/topics/1217364
   Helpful guide for learning accuracy from the highest ranking player with the highest ranked accuracy. Seriously, read this.
   
   How to improve in osu!mania: https://osu.ppy.sh/community/forums/topics/146615
   Good guide for beginners and intermediate players, Bible bashes 7k a bit but otherwise has some good advice.
7. Mirrors: (RIP Bloodcat)
   
   hentai.ninja: https://hentai.ninja/
   This is the current best option until chimu opens up. No DMCA claimed maps but everything else works just fine.
   
   chimu.moe: https://chimu.moe/
   Work in progress, hopefully starts up early February. Will have DMCA claimed maps and runs off of a fork of Cheesegull.

ESwox wrote:

The ERA Zenith's Comprehensive How-To-Accuracy Guide link is down

Jianshyy wrote:

Quaver also supports Etterna and Malody(IIRC) maps too.

As for skinning, you should also add Skinship too since they provide skinning tutorials and resources. Here's their website: https://tutorial.skinship.xyz
Also, they have a Discord server too. The people there are very helpful and very good at giving constructive feedback regarding skinning. Although most of people there usually make skins for standard, there are also people who are good at making mania skins too.