Extensions Warehouse at http://extensions.sketchup.com/ is the official extensions site. It is also available directly inside SketchUp and can be found by navigating to Window | Extensions Warehouse.

SketchUcation Plugin Store at http://sketchucation.com/resources/plugin-store-download. This is an extension for downloading and managing extensions, having the largest collection of SketchUp plugins online, with instant install, plugin sorting, and more useful features.

Smustard at http://www.smustard.com/. It is a collection of free and for-pay plugins.

SketchUp STL at http://extensions.sketchup.com/en/content/sketchup-stl converts SketchUp models into the common .stl format for 3D printing

Solid Inspector at http://extensions.sketchup.com/en/content/solid-inspector checks for errors preventing solid groups and components

CleanUp³ at http://extensions.sketchup.com/en/content/cleanup%C2%B3 automatically deletes stray lines and performs other tedious tasks

Import DXF at http://sketchucation.com/forums/viewtopic.php?f=323&t=31186 imports existing artwork in .DXF format

Edge Tools² at https://extensions.sketchup.com/en/content/edge-tools² cleans up imported .DXF and .DWG files

Make Faces at http://www.smustard.com/script/MakeFaces fills in imported linework with faces for pushing/pulling into 3D

Roundcorner at http://sketchucation.com/forums/viewtopic.php?t=20485 is used to add fillets to your 3D model, making the corners rounded

BezierSpline at http://sketchucation.com/forums/viewtopic.php?t=13563 is used to make complex curves

Joint Push Pull Interactive at http://sketchucation.com/forums/viewtopic.php?p=496773 is used to make wall thickness; it works best on models of simple to medium complexity

Add Terrain Skirt at http://sketchucation.com/forums/viewtopic.php?p=359903#p359903 quickly adds a flat base to terrain models, making them solid for printing

i.materialise 3D Print Service at http://extensions.sketchup.com/en/content/3d-print-service uploads a model directly from SketchUp to i.materialise

Curviloft at http://forums.sketchucation.com/viewtopic.php?t=28586 is used to create complex curved surfaces.

Bounding box at http://extensions.sketchup.com/en/content/draw-boundingbox draws faces on the bounding box of a group or component. It is useful for aligning odd-shaped objects.

Artisan priced $39 at http://artisan4sketchup.com/ adds subdivision capabilities inside SketchUp for organic modeling.