Commonly used to integrate with Tor
HTTP Proxies
Privoxy: an HTTP proxy that speaks SOCKS4a. Also does html/cookie scrubbing. (Win32, Linux, BSD, OS X)
Polipo a small and fast HTTP proxy that can be used instead of Privoxy.
3proxy Small proxy with a lot of features
SOCKS Client Wrappers
dsocks: SOCKS client wrapper with enough support to use the built-in OpenSSH or Tor local SOCKS proxy. (BSD, OS X)
tsocks: a program that intercepts connect() system calls and redirects them through Tor. Warning: the sourceforge version leaks DNS. Use the patched version. (Linux, BSD)
ProxyChains: like tsocks, features user-defined list of proxies and has full DNS support (no leaks). Warning: proxychains bypasses exit nodes' DNS resolvers. (Linux, BSD, Solaris)
SOCKS-supporting Relays
socat: multipurpose relay including SOCKS4a support. (Win32 under Cygwin, Linux, BSD, OS X)
connect: adds SOCKS support to SSH. (Win32, Linux, BSD, OS X)
sslredir: if your IRC server or news site supports SSL, you can send an encrypted connection through Tor even if your client application doesn't support SSL. (Linux, more?)
Windows SOCKS Forwarders
Freecap: redirects traffic to a SOCKS server. Graphical. (Win32)
Sockscap: redirects traffic to a SOCKS server. Graphical, proprietary. (Win32)
DNS Resolvers
- tor-resolve: turns hostnames into IPs privately via Tor. Shipped with the Tor package. (Win32, Linux, BSD, OS X)
Tor Controllers
TorK: an advanced Tor controller for KDE, with many features. Quite useful! (Unix/BSD/Linux)
Vidalia: a cross-platform Tor controller with GUI. Under development, so check it out! (Win32, OS X, X11)
Mozilla Extensions/Add-ons
Switchproxy: Mozilla extension to help you manage your proxy settings. (Firefox, Mozilla, Thunderbird)
Torbutton: Firefox extension to monitor and control Firefox's use of Tor. (Firefox)
Might also be useful but less tested
SOCKS-supporting Relays
On Linux and BSD, most utilities that aim to transparently intercept connections and redirect them through Tor have been obsoleted by Tor's TransPort.
Transproxy: Transparent proxy for HTTP requests with ipfw, ipnet, ipfwadm, ipchains or iptables. (FreeBSD, Linux).
Kernel socks bouncer: redirects certain streams into Tor. Doesn't deal with DNS. Linux 2.6 only.
Transocks: Transparent proxy to redirect traffic through a SOCKS proxy. Uses iptables. (Linux)
Windows SOCKS Forwarders
Torcap: similar to sockscap and freecap. Has different strengths/weaknesses. (Win32)
SOCKS Libraries
Antinat: seems to be a SOCKS client library (and server, but you'd only want the client side) that supports SOCKS4a.
DNS
Tor's 0.2.0.x series provides a built-in DNS forwarder, obsoleting TorDNS and dns-proxy-tor. ttdnsd might still be useful since it supports all query and RR types, but with negative implications for anonymity since it bypasses your current exit node's resolver.
TorDNS: a DNS server for Windows that runs on localhost and proxies queries through Tor.
dns-proxy-tor: a DNS server that uses Tor to map IP addresses to domain names. (Win32, Linux, BSD, OS X?)
TTDNSD - TOR TCP DNS Daemon: A DNS resolver for TOR that basically just bridges UDP to TCP. Provides full DNS over TOR which is especially usefull for running a transparent proxy. (un*x)
DNSEye - DNS Lookup Monitoring: A DNS monitor which displays DNS lookups and may be useful for detecting leaks. DNS lookups done though TOR should not be detected by this monitor. (Win32)
Mozilla Extensions/Add-ons
FoxyProxy: a Firefox extension/add-on that lets you configure all sorts of things, including using Tor only for certain hosts. This is currently worrisome because a website can intentionally ask you to load a page/image/etc from a host you're not proxying.
Virtual Machines
JanusVM: A virtual machine with transparent proxy of HTTP, TCP and DNS through Tor. (Win32)