centralised exchange signature

In exchange 2010 is built in.
For other versions I don't know any free programs.

Here you have some additional info:
http://www.howto-outlook.com/howto/corporatesignatures.htm

Here's the script we use in our environment.  It's VBScript, and it's simply a user based logon script.  When run, it queries a variety of attributes on an AD user object, and creates a sig file for Outlook.

This one collects a few different attributes, including department, phone, fax, mobile, etc.,  Should get you going pretty well.
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On Error Resume Next

Set objSysInfo = CreateObject("ADSystemInfo")

strUser = objSysInfo.UserName
Set objUser = GetObject("LDAP://" & struser)

strName = objUser.FullName
strTitle = objUser.Title
strDepartment = objUser.Department
strCompany = objUser.Company
strPhone = objUser.telephoneNumber
strPager = objuser.pager
stremail = objuser.mail
strFax = objuser.faxnumber
strmobile = objuser.mobile
strWeb = "www.yourdomain.com"

If lcase(right(stremail,8)) = "yourdomain.com" then

Set objWord = CreateObject("Word.Application")
Set objDoc = objWord.Documents.Add()
Set objSelection = objWord.Selection
Set objEmailOptions = objWord.EmailOptions
Set objSignatureObject = objEmailOptions.EmailSignature
Set objSignatureEntries = objSignatureObject.EmailSignatureEntries

objSelection.Font.Size = "12"
objSelection.Font.Name = "Garamond"
objSelection.Font.Color = "-587137025"
objSelection.Font.Bold = True
objSelection.TypeText strName
objSelection.TypeText Chr(11)
objSelection.TypeText strTitle
objSelection.TypeText Chr(11)
objSelection.Font.Bold = False
objSelection.TypeText "Company Name"
objSelection.TypeText Chr(11)
objSelection.TypeText "Another line of text"
objSelection.TypeText Chr(11)

if strPager <> "" then
objSelection.TypeText "Office: " & strPager
objSelection.TypeText Chr(11)
else
      if strPhone <> "" then
      objSelection.TypeText "Office: " & strPhone
      objSelection.TypeText Chr(11)
      end if
end if

if strmobile <> "" then
objSelection.TypeText "Mobile: " & strmobile
objSelection.TypeText Chr(11)
end if

if strFax <> "" then
objSelection.TypeText "Fax: " & strFax
objSelection.TypeText Chr(11)
end if

objSelection.TypeText "Email: "
objSelection.Hyperlinks.Add objSelection.Range, "mailto:" & strEmail,,, strEmail
Set objSelection = objDoc.Range(objSelection.End-Len(strEmail & " "),objSelection.End)
With objSelection.Font
      .Name = "Garamond"
      .Size = "12"
End With
Set objSelection = objWord.Selection


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How do i change font and colors and add logo for this?

<br>%%FirstName%%
%%LastName%%<br>
%%Title%%<br>
%%Company%%<br>
Phone:%%PhoneNumber%%<br>
Email:%%Email %%<br>

i cant use login script, because exchange is in different domain

In the above script, it's purely querying the Attributes of an Active Directory Account and uses fields if they're present on the AD object.  It doesn't matter which domain the Exchange server is on--at least I don't think it does.

To change the font, look for the word Garamond in the script.  Change it to your choice of font, and size.  The color portion is trickier.  The CHR(11) represents the color.  I'm not recalling the actual source chart of this color reference, and when I do change the color options, it's not always consistent depending on the os and version of Outlook on the client.

As davorin states, yep, plain html.  This script creates a few versions of your signatures, with an HTML version included.  Adding the code to the script though, is another story.  Since the script creates the multiple versions of the signature, the plain text signature(s) wouldn't be able to support the HTML logo.