Note: The table headings (positive/negative) are not necessarily meaningful.

Positive	Negative
acquire	release
add	remove (e.g. an item), subtract (arithmetic)
advance	retreat
allocate	deallocate (correct), free (common)
allow	deny
assemble	disassemble
assign	deassign (e.g. a task)
associate	dissociate
attach	detach
begin	end
bind	unbind
commit	rollback
compile	decompile
compress	decompress
connect	disconnect
construct	destruct
create	destroy
do	undo
enable	disable
encode	decode
encrypt	decrypt
enqueue	dequeue
enter	leave, exit
expand	collapse (e.g. a tree view), contract
first	last
freeze	unfreeze
front	back
get	set
grant	revoke
head	tail
import	export
include	exclude
increase	decrease
increment	decrement
indent	dedent
inflate	deflate
inject	eject
input	output
insert	delete (e.g. from a collection), extract (e.g. in a stream)
install	uninstall
link	unlink
load	unload (e.g. a module), save (e.g. state), store (e.g. a value)
lock	unlock
maximum	minimum
new	old
next	previous
open	close
compose	parse
paste	cut
push	pop (e.g. a stack item), pull (communication strategy)
raise	lower
read	write
reference	dereference (e.g. a pointer), unreference (e.g. an object in the case of reference counting)
register	deregister
resume	suspend
select	deselect
send	receive
serialize	deserialize
set	unset (e.g. a key), clear (e.g. a bit), get (e.g. a value)
set up	tear down
show	hide
start	stop
start up	shut down, clean up (maybe)
up	down
upper	lower

Remarks

Motivation

Using sensible, consistent wording in APIs improves readability, usability and greatly helps new users getting started. There is nothing more annoying than wondering what a function call is supposed to do while reading (someone else's or your own) code.

Initialize

There is no precise antonym for initialize and I don't think there has to be: By using initialize only for simple initializations that don't need any clean up code (initialize as its own antonym) you can

1. establish a convention through that initialization routines without a corresponding clean up routine can be easily recognized, and
2. circumvent the need for an antonym ;).

For routines that do not fall into this category, select another verb from the table above (most of the time there is a more descriptive alternative anyway).

If you still feel like you need an antonym; possibilities include but are not limited to: exit (very common in conjunction with init), reset, finalize, terminate.

"De-" vs. "Un-"

Both prefixes (de- and un-) can be defined as a reversal of action. Most of the time the prefix de- is used for verbs while un- is used for adjectives. There are many exceptions to this rule though. In some cases careful usage of these prefixes can avoid ambiguity. For example, "he was unregistered" is ambiguous because it can mean either "he was not registered" or "his registration was removed", while "he was deregistered" is not.