Here are some known third-party applets that are compatible with gnome-panel 3.14 and newer versions. For more applets, look at gnome-applets.
Indicator Applet — tray developed and used by Ubuntu.
Command Runner Applet — periodically displays a command output (we have a better replacement in gnome-applets now).
Sensors Applet — displays readings from hardware sensors, including CPU temperature, fan speeds and voltage (only git snapshot works).
Workrave — a program that assists in the recovery and prevention of Repetitive Strain Injury.
GNOME window applets — shows window title and buttons on the panel.
Miscellaneous:
uim input method framework has a gnome-panel applet (only git snapshot works; support was added in PR #48).
The talika-applet has some nice features similar to the Windows Vista/7 task bar, but is only GTK+2.