You can compile plugins within ImageJ using either the
Plugins>Compile and Run command or by pressing ctrl-r
(File->Compile and Run) in ImageJ's built in text editor.

The plugins contained in Input-Output>QuickTime_Plugins.jar
(Mac OS X only) are preinstalled as the File>Import>Using QuickTime,
File>Import>Video, File>Import>QuickTime Player and
File>Save As>QuickTime Movie commands.

The following third party plugins are included with ImageJ 1.45:

Analyze Reader/Writer (Guy Williams)
   File>Import>NIfTI-Analyze, File>Save As>Analyze 7.5...
   http://imagej.nih.gov/ij/plugins/nifti.html
   
Bezier Curve Tool (Berin Martini)
   Toolbar>>Bezier Curve Tool
   http://imagej.nih.gov/ij/plugins/bezier-tool/

Biorad Reader
   Plugins>Input-Output>Biorad Reader
   http://imagej.nih.gov/ij/plugins/biorad.html

Cell Counter (Kurt De Vos)
   Plugins>Analyze>Cell Counter    
   http://imagej.nih.gov/ij/plugins/cell-counter.html

Deltavision Opener
   Analyze>Input-Output>Deltavision Opener   
   http://imagej.nih.gov/ij/plugins/track/delta.html

DM3 Reader (Gregory Jefferis)
   Analyze>Input-Output>DM3 Reader   
   http://imagej.nih.gov/ij/plugins/DM3_Reader.html

Exif Reader (Gali Baler)
   File>Import>Exif Data   
   http://imagej.nih.gov/ij/plugins/exif-reader.html

Fast Filters (Michael Schmid)
   Plugins>Filters>Fast Filters
   http://imagejdocu.tudor.lu/doku.php?id=plugin:filter:fast_filters:start

Fit Polynomial (Michael Schmid)
   Plugins>Filters>Fit Polynomial
   http://imagejdocu.tudor.lu/doku.php?id=plugin:filter:fit_polynomial:start

ImageJ 3D Viewer (Benjamin Schmid)
   Plugins>3D>Volume Viewer
   http://imagej.nih.gov/ij/plugins/3d-viewer/

Interactive 3D Surface Plot (Kai Uwe Barthel)
   Plugins>3D>Interactive 3D Surface Plot
   http://imagej.nih.gov/ij/plugins/surface-plot-3d.html

Kalman Stack Filter (Christopher Philip Mauer)
   Plugins>Filters>Kalman Stack Filter
   http://imagej.nih.gov/ij/plugins/kalman.html

LSM Reader (Patrick Pirrotte)
   Plugins>Input-Output>LSM Reader
   http://imagej.nih.gov/ij/plugins/lsm-reader.html

Volume Viewer (Kai Uwe Barthel)
   Plugins>3D>Volume Viewer
   http://imagej.nih.gov/ij/plugins/volume-viewer.html

Zeiss ZVI Reader
   Plugins>Input-Output>ZVI Reader
   http://imagej.nih.gov/ij/plugins/zvi-reader.html