Requirements

Download the necessary files from here

Extract the jquery-autocomplete folder to your static folder

Include the necessary files in your model

response.files.append(URL(r=request,c='static/jquery-autocomplete',f='jquery.autocomplete.js'))
response.files.append(URL(r=request,c='static/jquery-autocomplete',f='jquery.autocomplete.css'))

Getting the items locally

Add the widget to your model

def autocomplete_widget(f,v):
    import uuid
    d_id = "autocomplete-" + str(uuid.uuid4())[:8]
    wrapper = DIV(_id=d_id)
    inp = SQLFORM.widgets.string.widget(f,v)
    rows = f._db(f._table['id']>0).select(f,distinct=True)
    itms = [str(t[f.name]) for t in rows]
    scr = SCRIPT('var data= "%s".split("|");jQuery("#%s input").autocomplete(data);' % \
                  ("|".join(itms),d_id))
    wrapper.append(inp)
    wrapper.append(scr)
    return wrapper

Getting the items remotely

If you want to fetch the items remotely using an ajax call then use this widget instead:

def autocomplete_widget(f,v):
    import uuid
    d_id = "autocomplete-" + str(uuid.uuid4())[:8]
    get_url = URL(r=request,c='default',f='get_items')
    wrapper = DIV(_id=d_id)
    inp = SQLFORM.widgets.string.widget(f,v)
    scr = SCRIPT('jQuery("#%s input").autocomplete("%s",{extraParams:{field:"%s",table:"%s"}});' % \
                  (d_id, get_url,f.name,f._tablename))
    wrapper.append(inp)
    wrapper.append(scr)
    return wrapper

And include this in your controller. WARNING: Alter this to take your own security needs into account. It is not secure as is.

def get_items():
    itms = []
    if request.vars.q and \
       request.vars.table and \
       request.vars.field:
        q = request.vars.q
        f = request.vars.field
        t = request.vars.table
        fld = db[t][f]
        rows = db(fld.upper().like(q.upper()+"%")).select(fld,distinct=True) 
        itms = [str(t[f]) for t in rows] 
    return '\n'.join(itms)

Assign the widget

db.things.name.widget = autocomplete_widget

Presto whammo! You're done.