There's nothing better than finding a library that does exactly what you're looking for with a clean and sane API. I'll gradually add to this posting as it strikes me.

Libraries I Like

Fabric "Fabric is a Python library and command-line tool designed to streamline deploying applications or performing system administration tasks via the SSH protocol." I wanted to get into Capistrano, but it's Rails-oriented and I do Python stuff. I could adapt it to Django, but why leave Python? Especially after the latest refactoring, Fabric does everything I want it to do and it does it well. I'm not really a shell scripter, so being able to use Python to slice-'n-dice in Fabric scripts is doubly nice for me.

chain.js This is javascript templating done right. Rather than the usual templating languages that are a mishmash of markup and javascript, this is a simple declarative way to splat data into your HTML.

flot A pure canvas/JS plotting library that is very flexible and generates pretty graphs. It's API is clean and very javascript-esque. One particularly nice feature that's not present in a lot of plotting libraries, but I use all the time, is handling times and dates.

Libraries I Haven't Tried--Yet

These libraries look interesting if I ever need them. I haven't tried them yet, so no endorsements, but they look good.

Synesketch OMG, so cool. It's a text textual emotion recognition and visualization engine. What does that mean? Have a look at some of Syneketch's pictures. I can't wait to make something with it!

Really Simple History "Really Simple History is a lightweight JavaScript library for the management of bookmarking and browser history in Ajax/DHTML applications."

Underscore "Underscore is a utility-belt library for Javascript that provides a lot of the functional programming support that you would expect in Prototype.js (or Ruby), but without extending any of the built-in Javascript objects."

eCSStender A Javascript library that enables extensions to CSS such as CSS 3 colors, selectors, and such--even for browsers that don't support them.

Modernizr "Modernizr is a small and simple JavaScript library that helps you take advantage of emerging web technologies (CSS3, HTML 5) while still maintaining a fine level of control over older browsers that may not yet support these new technologies."

ie7-js "A JavaScript library to make MSIE behave like a standards-compliant browser." This one sounds awesome.

Candygram "Candygram is a Python implementation of Erlang concurrency primitives. Erlang is widely respected for its elegant built-in facilities for concurrent programming. This package attempts to emulate those facilities as closely as possible in Python." I really like Erlang style concurrency so this library immediately appeals. On the downside, it's current implementation is rather heavyweight and it hasn't been updated since 2004. I can't get the repo without CVS... eww.

Flask "Flask is a micro web development framework for Python." There are lots of these but I like Armin Ronacher's style.

Hashdot "Hashdot elevates Java-platform script interpreters to first class status on Unix-like operating systems. It provides a script aware replacement to the stock java launcher, and thus avoids numerous issues in using the java launcher to bootstrap a script interpreter." When experimenting with Clojure I ended up writing my own shell script launcher to solve exactly this problem. I found this via technomancy.

Protovis "is a visualization toolkit for JavaScript using SVG. It takes a graphical approach to data visualization, composing custom views of data with simple graphical primitives like bars and dots." Pretty. The more ways to do pretty stuff via javascript, the better.

Ottoman "Ottoman is a lightweight, reliable key-value store with multi-version concurrency control."

Libtask "Libtask is a simple coroutine library... Libtask gives the programmer the illusion of threads, but the operating system sees only a single kernel thread. For clarity, we refer to the coroutines as "tasks," not threads."

libslack "Libslack is a library of general utilities designed to make UNIX/C programming a bit easier on the eye." There's lots of good stuff here. Particularly interesting are the tools for writing daemons, the network abstractions, and an agent orient programming system!