Where to get packages

OpenWrt does have a Package management system.
OpenWrt does offer a Software repository, which currently (2011-Feb) contains about 2000 packages. No, this is no typo.
other projects, offer additional repositories
the utility to handle the installation and removal of packages is opkg.

Official packages

The official OpenWrt packages are found in the respective packages folder on the repository server. Packages containing shell scripts only, are of course architecture-independent. All packages containing binaries are architecture-dependent, and some, e.g. kmods, are kernel version specific.

Each release has it's own directory containing sub-directories for the different platforms supported, each platform-directory contains pre-build images for the different targets and a packages directory containing the installable *.ipk files.

Here are some examples of OpenWrt package repositories for different compilation targets and OpenWrt versions. The specific repository for your OpenWrt version should already be configured correctly in OPKG when installing the specific OpenWrt image for your device.

For the OpenWrt 10.03 'Backfire' stable and brcm47xx platforms you find them here: http://downloads.openwrt.org/backfire/10.03/brcm47xx/packages/
For the OpenWrt 8.09.2 'Kamikaze' stable and ar7 platforms you find them here: http://downloads.openwrt.org/kamikaze/8.09.2/ar7/packages/
For the snapshot-release and ar71xx platforms you find them here: http://downloads.openwrt.org/snapshots/trunk/ar71xx/packages/. While the other repositories are mostly not updated, the packages in the trunk repositories are continuously built by the OpenWrt build bots. Beware with upgrading packages, since some (kernel) packages are made for the specific OpenWrt build built at the same time as the package.

Legacy 'White Russian' packages are still available: http://downloads.openwrt.org/whiterussian/packages/ and the Backport repository.
Source tar-balls for most packages can be found at http://downloads.openwrt.org/sources/ and http://mirror2.openwrt.org/sources/.

Creating custom packages

→ Howto Create a Package to create custom packages for OpenWrt using the toolchain (Buildroot)
→ crosscompile to simply sample a program

Third party packages

Third party packages are untested and unsupported by OpenWrt, and no warranties are made about their safety or usefulness. They could very well contain backdoors and such. So install only packages from sources, you trust! Please get support for third-party packages from the maintainers of those packages, not the OpenWrt developers.

Common sources for such packages include:

ipkg.be package tracker
NSLU2 "optware" - package feed targeted at devices with external storage

Usage of the OPKG package manager

→ opkg – Technical Reference. Below you see a very short outtake:

Command	Description
`opkg`	Print a list of all available commands and options
`opkg update`	Download the current list of packages available
`opkg list`	View the list of all packages in the current package list. Use with regular expressions
`opkg list_installed`	View the list of installed packages
`opkg install umurmur`	Install the package named `umurmur`, can also use a FQDN instead, e.g. opkg install http://downloads.openwrt.org/snapshots/trunk/ar71xx/packages/umurmur-polarssl_0.2.6-1_ar71xx.ipk
`opkg remove umurmur`	Remove the `umurmur` package

Option	Option long	Description
-V <level>	–verbosity <level>	Set verbosity level to <level>. 0 = errors only, 1 = normal messages (def), 2 = informative messages, 3 = debug, 4 = debug level 2
-d <dest_name>	–dest <dest_name>	Use <dest_name> as the the root directory for package installation, removal, upgrading. <dest_name> should be a defined dest name from the configuration file, (but can also be a directory name in a pinch).
-o <dir>	–offline-root <dir>	Use <dir> as the root directory for offline installation of packages.