Enable auto-complete via TAB in most modes through a defadvice

behavior.el

@@ -60,3 +60,10 @@
   (xterm-mouse-mode 1)
   (global-set-key [mouse-4] 'scroll-down-line)
   (global-set-key [mouse-5] 'scroll-up-line))
+
+;; Auto-complete when indenting.
+(defadvice indent-for-tab-command (around ac-before-indent activate)
+  "Call `auto-complete' if text was recently entered"
+  (if (ac-trigger-command-p last-command)
+      (auto-complete)
+    ad-do-it))

mode-customizations/ruby-mode.el

@@ -71,6 +71,13 @@
   (ruby-indent-line t)
   (end-of-line))
 
+;; Auto-complete when indenting
+(defadvice ruby-indent-command (around ac-before-ruby-indent activate)
+  "Call `auto-complete' if text was recently entered"
+  (if (ac-trigger-command-p last-command)
+      (auto-complete)
+    ad-do-it))
+
 ;; Yasnippet workaround for ruby-electric-mode
 ;; See: http://code.google.com/p/yasnippet/issues/detail?id=71
 (defun yas/advise-indent-function (function-symbol)