It’s very simple to use WordPress 3.8’s new icon font dashicons for a custom post type menu icon. When you register a post type, set the menu_icon argument to the CSS class of the dashicon you wish to use. Check out the reference below for a list of available classes.

Dashicons as menu_icon

// Testimonials management
register_post_type( 'testimonials', array(
    'public' => true,
    'menu_icon' => 'dashicons-format-status',
    'label' => 'Testimonials',
    'supports' => array( 'title', 'editor', 'custom-fields' )
) );

Dashicons Reference

Here’s a list of dashicons available for use in WordPress 3.8+. If you’re interested in specific character encodings check out Mel Choyce’s handy tool.

menu
admin-site
dashboard
admin-post
admin-media
admin-links
admin-page
admin-comments
admin-appearance
admin-plugins
admin-users
admin-tools
admin-settings
admin-network
admin-home
admin-generic
admin-collapse
welcome-write-blog
welcome-add-page
welcome-view-site
welcome-widgets-menus
welcome-comments
welcome-learn-more
format-aside
format-image
format-gallery
format-video
format-status
format-quote
format-chat
format-audio
camera
images-alt
images-alt2
video-alt
video-alt2
video-alt3
image-crop
image-rotate-left
image-rotate-right
image-flip-vertical
image-flip-horizontal
undo
redo
editor-bold
editor-italic
editor-ul
editor-ol
editor-quote
editor-alignleft
editor-aligncenter
editor-alignright
editor-insertmore
editor-spellcheck
editor-distractionfree
editor-kitchensink
editor-underline
editor-justify
editor-textcolor
editor-paste-word
editor-paste-text
editor-removeformatting
editor-video
editor-customchar
editor-outdent
editor-indent
editor-help
editor-strikethrough
editor-unlink
editor-rtl
align-left
align-right
align-center
align-none
lock
calendar
visibility
post-status
edit
trash
arrow-up
arrow-down
arrow-right
arrow-left
arrow-up-alt
arrow-down-alt
arrow-right-alt
arrow-left-alt
arrow-up-alt2
arrow-down-alt2
arrow-right-alt2
arrow-left-alt2
sort
leftright
list-view
exerpt-view
share
share-alt
share-alt2
twitter
rss
facebook
facebook-alt
googleplus
networking
hammer
art
migrate
performance
wordpress
wordpress-alt
pressthis
update
screenoptions
info
cart
feedback
cloud
translation
tag
category
yes
no
no-alt
plus
minus
dismiss
marker
star-filled
star-half
star-empty
flag
location
location-alt
vault
shield
shield-alt
search
slides
analytics
chart-pie
chart-bar
chart-line
chart-area
groups
businessman
id
id-alt
products
awards
forms
portfolio
book
book-alt
download
upload
backup
lightbulb
smiley