Spigot 1.8 will be released very soon!
More information here!
Here are over 50 Bukkit alternatives I listed, most of them are not yet fully functional or/and not intended for public use.
1. Sponge
A new Minecraft modding API combining the strengths of the server and client modding communities, a lot of people are involved (including sk89q, cpw, Spigot team, The Canary team, Glowstone(SpaceManiac) and a lot more).
You can also install Pore plugin in Sponge to support Bukkit plugins.
2. CanaryMod [IMG]
CanaryMod is the direct spiritual and code-wise successor of hMod.
Currently stable and has a ton of great plugins to install.
3. Glowstone
Glowstone is a lightweight, from scratch, open source Minecraft server written in Java that supports plugins written for the Bukkit API.
4. ServerForge
ServerForge is a Forge based Bukkit alternative, allows you to install Forge mods like Cauldron.
5. MCServer
A custom Minecraft server written completely in C++. It's a fast, lightweight, and awesome replacement for Vanilla.
6. Rush
An open-source Netty 5 based Minecraft server in Java.
7. IngotAPI
IngotAPI is similar to Bukkit's API to try to smooth the transition for developers moving from Bukkit.
8. StickyPiston
StickyPiston is a new API for Minecraft, Pushing Minecraft servers to their limit.
9. Rainbow
A custom Minecraft 1.8 server .jar where all blocks and entities work.
10. Wolf in a Bukkit
Wolf in a bukkit is intended to be an lgpl replacement for the gpl bukkit, to allow programs as craftbukkit and spigot to continue to exist.
11. Trident
TridentSDK is the successor to the Bukkit/Spigot projects.
12. Enderstone
Enderstone tries to make an open-source Minecraft server with custom API without using Mojang's server code.
13. Prismarine Standalone
A small minecraft server that aims to do minimum required for a basic Minecraft server.
14. Snowstorm
A New Minecraft server software.
15. GoldenApple
A powerful Minecraft server software.
16. Sink-MC
A Minecraft Server Variant.
17. Tripwire
A Minecraft Server Software.
18. Bitkit
A creative suite for a Minecraft server.
20. Bedrock
Bedrock is a Minecraft Server whose goal is to fully emulate vanilla Minecraft.
21. SkellyMC
The modular minecraft server.
22. Dandelion
Dandelion is trying to create a Minecraft server platform aiming to be less laggy than Bukkit or Spigot.
23. mcmd_prj
A Minecraft server program made in C++.
24. JungleTree
Open Source, lightweight implementation of a Minecraft server .
25. TBone
A Bukkit alternative.
26. Workbench
A Minecraft server API.
27. Mineserver
Custom Minecraft server software written in C++.
28. Rustymine
Barebones Minecraft server in Rust.
29. OverMC
Minecraft server developed in nodejs.
30. Mineqwaft
A Minecraft server written in Lisp.
31. CraftEmu
An open source Minecraft server emulator.
32. Granite
Granite - A different interface for Minecraft 1.8+.
33. N-API
Modification Development Interface with Forge.
34. Reflx
Open source Minecraft server plugin API, and reflection based implementation.
35. Stonebrick API
A Minecraft plugin API (Forge and Standalone).
36. MCPlusPlus
Minecraft Server in C++.
37. Cache
Cache is a modular server for Minecraft written in C++ utilising Boost and SQLite.
38. MCaap
Minecraft basic server.
39. PyServer
A minecraft server written in python.
40. Craft.Net
Minecraft server, client, and etc for .NET.
41. PrismarineMC
A new server platform.
42. Sandstone
A new Minecraft server software.
43. YARSS
Yet Another Rather Simple Server (YARSS) is an open source project by Tim Vosch.
44. MCAPI
Flexible Minecraft server.
45. MineSharp
An unofficial minecraft server.
46. MCPP
Multi-threaded, event-driven Minecraft server in C++.
47. RubyServer
Best server software for Minecraft there is! Compatible with the Bukkit API!
48. MineKit
Minecraft Server software that can run Bukkit plugins.
49. ProjectRails
Rails is a server client for Minecraft, which can run plugins and many more modded features.
50. Minecraft Ruby
Runnable Server Clone.
51. libMC.NET
.NET Library for Minecraft.
52. Minecraft Forge [IMG]
Modifications to the Minecraft base files to assist in compatibility between mods.
http://www.spigotmc.org/threads/over-50-bukkit-alternatives.29046/