Dean Attali started a GitHub repo listing RStudio addins, with the hope that other useRs will add to the list and make it a place to discover addins. Feel free to browse for useful addins, or make a pull request to add your own addin to the list. Here are the current items on the list:

NameDescriptionPackageOn CRAN?AuthorMore linksNotes
Colour pickerLets you easily select coloursshinyjsDean Attaliscreenshot,demo GIF
ggplot2 Marginal PlotsAdd marginal plots to ggplot2ggExtraDean Attaliscreenshot
ggplot Theme AssistCustomize your ggplot themeggthemeassistCalli Grossdemo GIF
Schedule RscriptsEasily scheduling RscriptstaskscheduleRjwijffelsscreenshot
Assign default valuesSet function arguments in selection to their default valuesjaddJenny Bryan
Wrap RmdWrap selected R Markdown text but don’t insert lines breaks into inline R codeWrapRmdTJ Mahr
Tidy DataInteractively build tidyr function call (gather)tidyshinyMango Solutions
Lattice PlottingInteractively build plots using thelatticesystemaddinplotsHomer White
Copy Frame to ClipboardCopy a data.frame to the clipboardcopydatBay Area Air Quality Management DistrictDoesn’t work on Windows
Render Rmd in ConsoleRender an R Markdown document in the global environmentRStudioConsoleRenderJeff Johnstonblog post
Document ThisAuto-generate Roxygen skeletons for functions and datadocthisMatthew Lincoln
Hist Add-InInteractively create histograms with ggplot2 and obtain the R CodelimoaddinStanislaus Stadlmann
Addin ManagerInstall and remove addinsaddinmanagerColin GillespieBlog post