Sometimes you want to modify the UI that is presented to users just a little, or in ways that the core permission system doesn't quite give you enough control over.
This review will address only the node edit screen and modifications you may want to make to it. Revamping the entire User Experience using Dashboard or Control-panel-like utilities is not covered.
The list of available Utility and Administration modules can be a bit long and wide to sort through to find just what you want.

Tasks (common or otherwise) I'll look for include:

• Removing, reducing some irrelevant elements to de-clutter the UI.
• Suppressing the 'revision information' or 'input format' box.
• Changing the words on the "Submit" buttons, or the descriptive help text on elements that can't be done through the UI.
• Change the size or weight of some fields.

Alternatives

Do it in CSS

Often it is possible to just "remove" some ugly form element from the flow using CSS. This may be the easiest option, though it could lead to head-scratching for later maintainers. DOCUMENT THIS.

Do it in Theme

Although it takes a bit of code, you may be able to find the appropriate 'theme' function that relates to the bit you want to change, eg theme_upload_form_current() to change how uploaded attachments are displayed. For coders only

Do your own form_alter

Making your own site-specific module is actually a lot easier than you'd think, so it's quite reasonable to make a module of your own that does nothing except modify one part of one form because you want it 'just so'. Some of the modules below started off like that before they were released to the public.

NOTE that most of these achieve their effect by simply "Hiding" unwanted elements. This is not secure as it can be overridden from the user end and it is also potentially frustrating in the long run for later administrators who expect certain options to be available.
These tweaks are mostly visual or design workarounds that override expected or designed behaviors, and should be overridden only if you can't convince your designer otherwise. Or, as an admin user you just don't want to look at a feature you never use but can't turn off

Modules

• Vertical Tabs is an entire UI overhaul of the node edit screen, and could solve many of your UX issues. It basically collapses the entire node form into much more manageable 'tabs' along the left instead of the default 'collapsing' fieldsets. This was so good it went into D7 core.
• Simplify Node Add is a good start, although it's a little limited in what it lets you modify, only a few core elements.
• Node and Comments Form Settings Looks like the freindliest of the bunch, containing a lot of useful options, like "Hide the Input Format fieldset"; "Option to remove revision log field"; "control the size of the body textarea field"
• FormFilter is probably the most useful. It's designed to offer hide/show modifications on elements in any form type, plus includes an override to allow admin users to selectively see them if they want.
• Form Alter UI Appears to directly expose the Drupal "Form Alter" options through a UI. (unreviewed)
• Fixcore is another grab-bag of (mainly UI) tweaks, including "Collapse the Revision information fieldset for new nodes" and "Make multiselect definition lists auto-sized"
• Auto NodeTitle will allow you to obscure/remove the nodes title. Although all nodes must internally have a title for management reasons this can be auto-generated and unused.
• Better Formats just provides extra control over the 'input format' box, adjusting the visibility and default setting on a per-user, per-content type basis.
• Unrelated (but maybe what you want instead).
  Form Tweak has no releases, listed here for completeness. Tweakbox is a dead D5 project that does not actually contain any form tweaks. Drupal tweaks is a much lower lever admin tool, and does not generally affect the UI. Form Controller and Form Module are low-level API utilities, and are unrelated to this job.