This has been brought up a few times and I wasn't able to find this documented anywhere. If you remove one of the default Clearspace widgets you have to enter the full class name to add the widget back. If you don't have access to the source code this can be somewhat difficult to figure out. Here is a full list of the available default widgets as of 2.5:

com.jivesoftware.community.widget.impl.CommunityActionsWidget
com.jivesoftware.community.widget.impl.CommunityProjectsWidget
com.jivesoftware.community.widget.impl.FormattedTextWidget
com.jivesoftware.community.widget.impl.HtmlTextWidget
com.jivesoftware.community.widget.impl.LatestPollWidget
com.jivesoftware.community.widget.impl.MembersListWidget
com.jivesoftware.community.widget.impl.PopularBlogPostsWidget
com.jivesoftware.community.widget.impl.PopularDocumentsWidget
com.jivesoftware.community.widget.impl.PopularThreadsWidget
com.jivesoftware.community.widget.impl.PrivateMessagesWidget
com.jivesoftware.community.widget.impl.ProjectActionsWidget
com.jivesoftware.community.widget.impl.ProjectCalendarWidget
com.jivesoftware.community.widget.impl.ProjectCheckPointsWidget
com.jivesoftware.community.widget.impl.ProjectStatusWidget
com.jivesoftware.community.widget.impl.ProjectTasksWidget
com.jivesoftware.community.widget.impl.RecentActivityWidget
com.jivesoftware.community.widget.impl.RecentBlogPostsWidget
com.jivesoftware.community.widget.impl.RecentContentWidget
com.jivesoftware.community.widget.impl.RecentDocumentsWidget
com.jivesoftware.community.widget.impl.RecentThreadsWidget
com.jivesoftware.community.widget.impl.RelatedSpacesWidget
com.jivesoftware.community.widget.impl.RssSubscriptionWidget
com.jivesoftware.community.widget.impl.SocialGroupActionsWidget
com.jivesoftware.community.widget.impl.SocialGroupOverviewWidget
com.jivesoftware.community.widget.impl.SubCommunitiesWidget
com.jivesoftware.community.widget.impl.TagCloudWidget
com.jivesoftware.community.widget.impl.TagGroupWidget
com.jivesoftware.community.widget.impl.TagWidget
com.jivesoftware.community.widget.impl.TopMembersWidget
com.jivesoftware.community.widget.impl.UserWidget
com.jivesoftware.community.widget.impl.WhosOnlineWidget
com.jivesoftware.community.widget.impl.YourFriendsWidget
com.jivesoftware.community.widget.impl.YourProjectsWidget
com.jivesoftware.community.widget.impl.YourSocialGroupsWidget
com.jivesoftware.community.widget.impl.YourStatusUpdatesWidget
com.jivesoftware.community.widget.impl.YourTasksWidget

Re-installing a widget

Please see this screencast for an example of re-installing the HTML widget: http://screencast.com/t/1iBZLCdHJ