1. Extending the DeskbarApplet
The whole point of the DeskbarApplet is that it's an all-in-one, "smart" search bar. Type in what you're looking for, and it should know whether you mean a web page or an e-mail address or something else.
To help it being "smart", you can write your own modules, or plug-in. In the 2.20 release cycle a brand new API has been introduced. A detailed instruction is available in Writing new-style modules for Deskbar-Applet
1.1. Process
All you have to do is go to preferences and drag your module file on the list of modules. That's it! You should then be able to, from your Deskbar, go to preferences, and activate your Handler. The DeskbarApplet monitors for new files in that directory, and compiles any Python it finds. No need for messing around with Makefiles, or configuration. If this automagical installation doesn't work for you, then it's a bug!
In fact, you should be able to drag .py or .bz2 file links directly from your browser into the Preferences window and they should be automatically downloaded and installed.
2. Third Party Handlers
If you've written your own DeskbarApplet Handler, add a link and a comment below.
2.1. Post-2.20 API
These modules only work with Deskbar-Applet >= 2.20
Tracker, searching with Meta Tracker
Gajim, Search and start conversations with buddies on your Gajim roster.
HostLookup, Very simple plugins that does IP/hostname lookups.
Twitter, Post updates to Twitter.com.
Wayd(What Are You Doing), Post updates to multiple micro-blogs at once, Twitter and Jaiku supported, more coming.
ekiga, Type in anything that starts with 'sip:' and you can call the address you entered with ekiga (sebp)
Google Suggest, It makes use of Google Suggest and just like the live search feature in Firefox the module will contact Google to suggest search terms to you (sebp) (requires python-simplejson package to be installed)
Wikipedia Suggest, You type in something and the module will suggest Wikipedia articles for you (sebp)
Remember The Milk, search your tasks stored at rememberthemilk.com (sebp)
SmsSend, send sms using smssend and Deskbar.
Runshell, executes a shell script from a given directory
GnomecatalogSearch, search in gnomecatalog catalogs
LEO dictionary lookup, searching http://dict.leo.org/
Converter, convert quantities from one unit to another ($50000 in pounds, 43 feet in metres, 20 minutes in microfortnights). (StuartLangridge, JohannesBuchner); Can be combined with Calculator extension 1.8.
Todotxt(fr),(en), search, add and modify items in your todo list. This plugins is based on the todotxt tools. (ThibauldNion).
onet.pl dictionary lookup, searching polish/(english|german/french) dictionaries provided by onet.pl.
MPD: Search and start playing music in the current playlist of a running MPD daemon. This is an updated version of the pre-2.20 API MPD plugin.
Pidgin Buddies: Start typing a buddy's name and get chatting. This is an updated version of Gaim Buddy List.
G-MAN: A simple plugin to search through the available MAN pages on the local filesystem, and display them using the GNOME documentation browser (Yelp).
Open Location: Open a directory from the path given in the query, and if the query looks like an incomplete path, proposes some completions (in the list of matches). Grants a behaviour similar to Nautilus' Ctrl+L, though not quite as good (no 'inline' completion). (ThibauldNion)
Contact Lookup: Addressbook search and contact display in the style of contact-lookup-applet. (FrederikElwert)
Wordreference.com translation: Translate a word into foreign language using Wordreference.com. (GuillaumeBour)
VirtualBox: launch virtual machine by typing name. (SunNing)
2.2. Pre-2.20 API
These modules only work with Deskbar-Applet <= 2.18
Deskbar Timer - a plugin that gives simple named countdown timer capability. I've also written a Tutorial on creating Deskbar Applet handlers.
Gmail Deskbar, a plugin to search Gmail (StuartLangridge)
SSH, a plugin to open a terminal with a new SSH session in it. Parses ~/.ssh/known_hosts and ~/.ssh/config to offer smart choices.
ssh plugin: a simplified version of the same, that respects the desktop terminal handler and doesn't require extra scripts. (EdCatmur)
Gaim Buddy List, a plugin to start a conversation with an online Gaim buddy. (SegPhault)
MPD, a plugin to parse the current playlist or database of a running MPD daemon.
Bugzilla Lookup, a plugin to search Bugzilla databases such as Mozilla
RAE's dictionary, a handler to look up Spanish words in the RAE's dictionary (JaimeFrutosMorales).
Gtkhtml RAE, Gtkhtml version of Jaime's RAE handler (SergioRubio)
Creative Commons Search, a handler to search for Creative Commons licensed content on Google, Yahoo and Flickr (JaimeFrutosMorales).
Tracker, searching with Meta Tracker
Calculator (version 1.6), do simple calculations with the deskbar applet. For when you want the answer fast.
Remember The Milk, handle tasks using Remember The Milk's Twitter interface. (PaulSwartz)
DoodleSearch, Search the Doodle index.
LDAP plugin, a handler for searching into LDAP directories.
Thunderbird address book, a handler for searching Thunderbird address book (build upon a custom mork python parser).
2.3. Obsolete
Old handlers that have been integrated in deskbar-applet core distribution:
Firefox History, a plugin to search inside the Firefox history. It has been integrated in deskbar core in the regular mozilla/firefox handler.
Delicious Deskbar, a plugin to search del.icio.us, the social bookmarking service (StuartLangridge)
Deskbar-Tomboy 0.1, search your Tomboy notes with Deskbar. (Replaced by new Tomboy module since 2.22)
Calculator (version 1.8), do simple calculations with the deskbar applet. For when you want the answer fast. (JohannesBuchner) (previous version 1.7 by Callum). Part of Deskbar core since version 2.23.4.